Final answer:
The best word to complete the sentence is 'es'.
Step-by-step explanation:
The best Spanish word to complete the sentence would be 'es'. When translated to English, the sentence would read 'Marisol is the best.' Here, 'es' is the third person singular form of the verb 'ser', which means 'to be'. In this context, it is used to describe Marisol as the best.
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